the default desktop

I didn't use Unity, but it wouldn't surprise me that substantial portions of GNOME were still used (GNOME is more than mutter). I believe there are less than 6 working implementations of the all daemons needed for a semi-standard DE, and I suspect that in more cases than we'd like, only 2 (GNOME, KDE, XFCE, LXDE/LXQT and Enlightenment are the only DEs I know of which could support separate implementations, and I wouldn't surprise me that XFCE, LXDE/LXQT and Enlightenment either use GNOME or KDE daemons, or don't implement them).

That latter part (needing to use a single i.e. GNOME implementation), and GNOME's backwards-compatibility breaks, probably create a fair number of unhappy users/developers for GNOME. I still run bits of GNOME (such as nm-applet), even though I'm using a tiling WM (awesomewm).